ELECTRONIC THEORY OF MATTER 287 



are such that each possesses twenty-three times as many 

 electrons as hydrogen has, we call it sodium. If each atom 

 has two hundred times as many as hydrogen, we call it lead 

 or quicksilver. If it has still more than that, it begins to 

 be conspicuously radioactive. 



It would seem as if the excessive radiation which follows 

 upon an overcrowded condition were caused by the prob- 

 ability of collision or encounter between the parts of an 

 atom; just as every now and then among the stars in the 

 sky two bodies encounter each other, and a great blaze of 

 radiation, or temporary star, results. Even in atoms of 

 which the parts are sparsely distributed such occurrences 

 are not impossible, though they are less frequent, and 

 accordingly it is to be expected that every kind of matter 

 may be radioactive to a very small extent ; a probability 

 which is now justified for most metals, by direct experiment 

 with very sensitive means of detection. 



Indeed, so far as radiation necessarily accompanies any 

 change of motion of an electron, and in so far as in every 

 atom some electrons are describing orbits and are therefore 

 subject to centripetal acceleration, a certain amount of 

 atomic radiation is inevitable, on the electric theory of 

 matter. In most cases it is imperceptibly small, but it must 

 be there, and accordingly an atom must be slowly under- 

 mining its own constitution by the gradual emission of its 

 internal or intrinsic energy in the form of ether-waves. 



Thus, then, it is reasonable to expect that, every now and 

 then, an atom will break up or collapse or divide into parts. 

 This process has been observed by Rutherford, of Montreal. 

 The radiation from many of the radioactive substances, on 

 being analyzed by a magnet, is found to be separable into 

 three parts : 1, the so-called ft rays, which are the shot-off 

 electrons already mentioned ; 2, some y rays, which appear 

 to represent an ethereal pulse an analogue as it were of 

 the sound-wave caused by the explosion or act of firing ; and 

 3, more important than either, a third kind of projectile 

 called the a rays, which are newly-formed atoms of foreign 



